The present invention relates to a method of determining the concentrations of components of a ternary mixture essentially consisting of sulfuric acid, hydrogen fluoride, and water and to a process for continuous production of hydrogen fluoride which comprises using the method.
Any method for determining the concentration of a given component of a mixture comprising a plurality of chemical substances of known kinds indirectly and expediently with a method other than direct determinations is expected to find application in many fields. For binary mixtures, several practical methods are known but when it comes to a ternary mixture, no effective method has been available.
Therefore, in determining the concentration of water in a ternary mixture containing water, for instance, it is common practice to sample the ternary mixture and quantitate the water therein with a direct method such as Karl Fischer""s method.
However, the direct method for quantitating a component of a ternary mixture is not only generally time-consuming and unsuited for applications requiring rapid determinations but often involves fairly large errors.
Furthermore, when such a ternary mixture contains a substance harmful to human beings, the direct determination and the necessary sampling have the disadvantage of involving risks for health hazards.
Meanwhile, hydrogen fluoride is a starting material of great importance for the production of various fluororesins and fluorine-containing compounds. Commercially, hydrogen fluoride can be obtained by reacting fluorspar with sulfuric acid. In the commercial production of hydrogen fluoride with this reaction, a high-boiling mixture containing unreacted sulfuric acid is not treated off-line but recycled for reuse.
The process in common use today comprises the following steps.
Thus, the process is a continuous production process comprising (1) a step of reacting the material fluorspar with starting sulfuric acid, (2) a step of separating the crude reaction product obtained in step (1) into (a) a low-boiling mixture composed predominantly of hydrogen fluoride and (b) a high-boiling mixture composed predominantly of unreacted sulfuric acid and containing small proportions of hydrogen fluoride and water, (3) a step of purifying and isolating hydrogen fluoride from said low-boiling mixture (a), a step (4) of adding a substantially equivalent amount of sulfuric anhydride to said high-boiling mixture (b) to form sulfuric acid through reaction with water to thereby convert substantially all the water to sulfuric acid and returning it together with unreacted sulfuric acid as sulfuric acid feed back to said step (1), and a step (5) of adding sulfuric acid in a supplemental amount to make the necessary amount of sulfuric acid available for reaction with the fluorspar.
The above steps are now described in further detail.
In the above step (1), which includes a kneading and heating stage, the starting fluorspar and sulfuric acid are introduced into a reaction system comprising a kneader and a kiln through respective the ducts and mechanically kneaded and reacted in the kiln under heating at 400 to 500xc2x0 C. This reaction between fluorspar (CaF2) and sulfuric acid (H2SO4) proceeds as follows to give hydrogen fluoride.
CaF2+H2SO4xe2x86x92CaSO4+2HF
Concurrently, the trace impurities in the material fluorspar, i.e. silicon oxide and calcium carbonate, give rise to water and other byproducts as follows.
SiO2+4HFxe2x86x92SiF4+2H2O
CaCO3+H2SO4xe2x86x92CaSO4+H2O+CO2
In order to provide hydrogen fluoride of high purity, a step of removing the unreacted feed and those byproducts is required.
Therefore, in the above step (2), the reaction product crude gas from said mixing and heating stage is fed to a crude gas washing column for washing and primary purification. This crude gas washing column has somewhat dissimilar functions between its upper zone and lower zone. Thus, the high-temperature crude gas generated from the reactor is first introduced into the bottom of the crude gas washing column and a high-boiling mixture (b) composed predominantly of sulfuric acid and accumulated in a reservoir tank disposed below said crude gas washing column is pumped up into the lower zone of the column at a level slightly below its middle portion to cause it to contact the ascending high-temperature crude gas to thereby remove the dust entrained from the reaction stage. Meanwhile, in the upper zone of the column, the liquid hydrogen fluoride available on condensation in a downstream stage and introduced from the column top is caused to contact the ascending dust-free crude gas to thereby remove a high-boiling fraction (sulfuric acid and water) through a condensation effect, while the low-boiling mixture (a) composed predominantly of hydrogen fluoride is withdrawn from the top of said crude gas washing column.
Then, in step (3), the low-boiling mixture (a) withdrawn in the above washing stage is purified to give the objective hydrogen fluoride of high purity.
The sulfuric acid occurring in the high-boiling mixture (b) separated in the above washing stage can be reused in the hydrogen fluoride-forming reaction for effective utilization of resources and should not be discarded for avoiding environmental pollution. Therefore, a step for recycling it as starting sulfuric acid is required.
Therefore, in the above step (4), provided is a stage in which the high-boiling mixture (b) separated in the above-mentioned washing stage is introduced into a mixing tank, while sulfuric anhydride (usually in the form of fuming sulfuric acid) is added to the tank so as to cause it to react with the byproduct water to give sulfuric acid, and fresh sulfuric acid is supplementarily added in an amount sufficient to make a substantially total necessary amount of starting sulfuric acid available for reaction with the material fluorspar for feeding to said kneading and heating stage.
In the stage described just above, when the amount of water is too large in comparison with sulfuric anhydride, the water in the sulfuric acid feed retards the reaction and the reaction product tend to form deposits and cakes. At the same time, corrosion of the equipment and pipeline proceeds throughout the whole recycling process. Therefore, in order that the amount of water will not be excessive, the amount of sulfuric anhydride to be added to the mixing tank must be accurately determined according to the amount of water occurring in the high-boiling mixture (b). As an important factor, the concentration of water in said high-boiling mixture (b) must be accurately determined in the above step (4).
Heretofore, this determination of water content has been carried out directly by sampling the high-boiling mixture from the duct constructing the production system at timed intervals and determining the concentration of water by Karl-Fischer""s or other method.
However, such method has the disadvantage that the sampling procedure itself involves handling of hydrogen fluoride and sulfuric acid, both of which are dangerous substances. Moreover, there is a fair time lag before acquisition of data following said sampling and this failure to generate real-time information results in being late in accurately controlling the amount of supply of sulfuric anhydride.
Developed in the above state of the art, the present invention has for its object to provide a method of determining the concentration of any given component of a ternary mixture which is rapid, involving minimal errors, and safe procedure-wise and to provide a process for continuous production of hydrogen fluoride which comprises using the method.
The method of the invention, designed to accomplish the above object, is a method of determining the concentration of each component of a ternary mixture essentially consisting of sulfuric acid, hydrogen fluoride, and water, which comprises measuring at least one set of the three physical quantities, namely (1) temperature, (2) ultrasound propagation velocity, and (3) electrical conductivity or viscosity, of the ternary mixture and converting measured values to the concentrations of the respective components according to calibration curves representing the relationships of the concentrations of respective components of a ternary mixture composed of sulfuric acid, hydrogen fluoride, and water with the above-mentioned three physical quantities as separately constructed beforehand.
The concentration determination according to the invention can be carried out typically in the following manner. Thus, for example, while the ternary mixture to be analyzed for the concentration of any component thereof is maintained at a given temperature, its ultrasound propagation velocity and either electrical conductivity or viscosity are measured and the measured values are converted to the concentration of the corresponding component with reference to the above-mentioned calibration curves.
The process of the invention for continuous production of hydrogen fluoride comprises (1) a step of reacting starting fluorspar with starting sulfuric acid, (2) a step of separating a crude reaction product as obtained in step (1) into (a) a low-boiling mixture composed predominantly of hydrogen fluoride and (b) a high-boiling mixture composed predominantly of unreacted sulfuric acid and containing small proportions of hydrogen fluoride and water, (3) a step of purifying and isolating hydrogen fluoride from said low-boiling mixture (a), (4) a step of adding sulfuric anhydride to said high-boiling mixture (b) in a substantially equivalent amount with respect to the water to convert substantially all the water to sulfuric acid and returning it together with said unreacted sulfuric acid as sulfuric acid feed to said step (1), and (5) a step of adding sulfuric acid in a supplemental amount to provide the amount of sulfuric acid needed for reaction with starting fluorspar, wherein, in said step (4), the amount of water occurring in said high-boiling mixture (b) is determined by the method of the invention for determining the component concentration, particularly the concentration of water, of a ternary mixture.